It is known to control air/fuel ratios of individual cylinders of a multiple cylinder engine using a single exhaust gas sensor such as an oxygen sensor. An example of such a system is the Individual Cylinder Fuel Control (ICFC) marketed by Delphi Corporation headquartered in Troy, Mich. It has been discovered that it would be advantageous for such a system to detect certain performance characteristics such as changes to the sensitivity of an oxygen sensor to variations in exhaust gas composition, or time shifts of an oxygen sensor output signal caused by, for example, changes to the physical location of the exhaust oxygen sensor relative to each cylinder of the engine. Furthermore, a suggested interpretation of the California Air Resources Board (CARB) regulations is that a sensor used for an input signal to another diagnostic or emission control strategy must be monitored or verified to be sufficient for that use.